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Orbital variation of the X-ray emission from the double neutron star binary J1537+1155

We observed the double neutron star binary (DNSB) containing PSR J1537+1155 (also known as B1534+12) with the Chandra X-ray Observatory. This is one of the two DNSBs detected in X-rays and the only one where a hint of variability with orbital phase was found (in the previous Chandra observation). Our follow-up observation supports the earlier result: the distribution of photon arrival times with orbital phase again shows a deficit around apastron. The significance of the deficit in the combined dataset exceeds 99%. Such an orbital light-curve suggests that the X-ray emission is seen only when neutron star B passes through the equatorial pulsar wind of neutron star A. We describe statistical tests we used to determine the significance of the deficit, and conclusions that can be drawn from its existence, such as interaction of the pulsar wind with the neutron star companion. We also provide better constrained spectral model parameters obtained from the joint spectral fits to the data from both observations. A power-law successfully fits the data, with best-fit photon index Gamma=3.1pm0.4 and unabsorbed flux f=(3.2pm0.8)x10^-15 erg s-1cm-2 (0.3-8 keV range).